The invention relates to an apparatus for applying thin layers to a substrate, by sputtering a cathode mounted target in a vacuum chamber.
To ensure a stable sputtering process at the desired working point, it is necessary to control the cathode power supply very precisely while keeping the gas feed constant. The simplest automatic control modes for this purpose are those based on current, voltage, and power. These automatic controls alone, however, are not sufficient to operate a sputtering cathode over a prolonged period of time in the critical region of the current-voltage characteristic (e.g., in the transition range between metallic mode and oxide mode).
It has therefore already been proposed that the amount of power be kept constant while the gas feed is automatically controlled with the help of a plasma emissions monitor (DD 271 827 13). In this known device for stabilizing operation, consisting of a plasma emissions monitor with a measurement system for converting an optical signal into an electrical signal and a common power supply with a switching device for pulsed operation of the plasmatron, each plasmatron has its own measuring system, the outputs of which are connected by way of holding stages to a superposition device. The switching device is connected by way of control stages to the superposition device, and on the output side the plasma emissions monitor and the valve for the reaction gas inlet are connected to the superposition device.
A method is known for monitoring the vapor deposition rate (DAS 27 00 979) and/or the composition of the material to be deposited during a vapor deposition process under vacuum in which a portion of the material to be vapor-deposited flows through a measurement zone, where the material to be vapor-deposited is exposed to radiation. The type of radiation is selected so that the electrons of at least some of the atoms of the material to be deposited flowing through the measurement zone are raised to a higher energy level. The photons created when the electrons fall back to the lower energy level are then recorded as a measure of the vapor deposition rate or as a signal providing information on the composition of the material to be vapor-deposited.
These known automatic control systems based on the use of a plasma emissions monitor or on the method according to DAS 27 00 979 are very expensive to produce, are very sensitive to reflections of foreign light, demand very precise adjustment, and are highly sensitive to uncontrolled dopings of the sputter gas.
These optical measurement methods also suffer from a decisive disadvantage in the case of systems which apply coatings under continuous operating conditions. Namely, they require a window to allow some of the light to pass through. This window is coated by stray vapor, as a result of which the optical properties of the window and thus the measurement values are changed.
The present invention avoids the disadvantages of the known automatic control apparatuses and in particular can be produced cheaply and operate reliably over a long period of time.
A measurement sensor, such as a potentiometric measurement electrode, compares the amount of a gas in the vacuum chamber or in a feed line connected to the vacuum chamber with a reference gas and sends the obtained signal or the potential difference being reached to the automatic control unit, which contains a signal amplifier. The control unit then drives the generator of the power supply.